Photodynamic therapy (PDT) is a promising new cancer intervention that is based on the photoactivation of a systemically administered photosensitizing dye. Absorption of visible light by the dye results in the photosensitized formation of singlet oxygen (1O2) and subsequent cytotoxic reactions of 1O2 with cellular substrate. Considerable effort has been devoted to elucidate the mechanisms by which these photosensitized oxidations contribute to tumor destruction. These studies proposed here are designed to continue and extend our efforts, begun 10 years ago, to elucidate the biochemical basis for PDT-induced cytotoxicity and to use the new knowledge acquired in this effort to enhance the efficacy of the therapy. The proposed studies are interdisciplinary in nature, applying the expertise of scientists in biochemistry, biophysics, radiology and physics. Important new questions have arisen during the course of the previous grant period. Modifications in the photoradiation regimen have been shown to be capable of producing significant differences in inhibition of tumor growth. The mechanisms underlying these effects remain to be determined and experiments designed to accomplish this form an important focus for the proposed renewal. Different xenografts grown in the same host (nude mouse) possess significantly different sensitivities to PDT/ This finding has raised important questions concerning mechanism of action, sensitizer uptake and distribution, intrinsic differences between tumor cell types, etc. These studies will be pursued as will closely related problems in primary and transplanted rat mammary tumor models. Five specific aims have been formulated: (1) to optimize the drug and irradiation regimen in PDT of the R3230AC tumor in vivo and to quantitate an optimum therapeutic ratio in that system; (2) to determine sensitivities of primary and transplantable NMU tumors in the same host and to determine mechanisms differences in sensitivity between the R3230AC and the human mesothelioma xenograft in the nude mouse; (4) to perform several direct experimental tests in vitro and in vivo of the mechanisms whereby fractionated and reduced dose rate irradiation enhance the therapeutic response; and (5) to investigate the problem of recurrent tumors in PDT. In all of these studies, biochemical methods will be used in close conjunction with NMR imaging and spectroscopy, fluorescent probes, FACS, and radioactive labeling techniques.